Although moisture from precipitation does not usually affect an astronomical binocular, condensation can affect it. In terms of frequency of occurrence of damage, condensation is by far the most harmful source of deterioration of astronomical binoculars. Condensation on the external optical surfaces results in being wiped more frequently than would otherwise be necessary, with the attendant damage that all too often accompanies frequent cleaning. (To protect against condensation in use, see the section on dew prevention in Chapter 7.) Condensation on internal optical surfaces may lead to inexpert dismantling of the binoculars, which itself can lead to damage, in an attempt to gain access to and clean the affected surfaces. The water itself accelerates the corrosion of the metal parts of the binocular, especially if there are places where two different metals are in contact. This corrosion leads to stiffness in moving parts and this stiffness, allied to the corrosion, accelerates the wear of these parts. Moist surfaces are a sine qua non for the growth of algae and fungi; so if you keep moisture at bay, you will keep these flora at bay also.
Much of this condensation damage ultimately results from failing to cap the binoculars at the end of an observation session. The cold binocular is taken into a relatively warm and humid place, where moisture from the air condenses on the
Figure 5.1. Eyepiece rainguard. The rainguard attaches to the strap and can be slipped over the eyepieces when the binocular is not being used.
colder surfaces of the binocular. Lens caps and cases then act to hold the moisture in place. It is a good practice to bring the binoculars indoors uncapped, place them horizontally on a firm flat surface in the room in which they are to be stored until they have reached thermal equilibrium, then cap them and put them in their cases. It is an even better practice to cap them outdoors before bringing them in unless the surfaces have been affected by dew.
Ideally, binoculars should be stored in a cool dry place; certainly not one that is subject to great fluctuations of temperature and humidity. I store mine in a closet in an unheated part of the house. Cases and caps help to guard against arthropods that might otherwise take up residence. Multipurpose "grab-and-go"binoculars are often stored, uncased and uncapped, on interior windowsills, where they are instantly available should an object of interest be spotted. They are placed resting on their objective ends, so the objective lenses are protected to some extent. On the other hand, the eyepieces of such binoculars act as dust magnets and soon show the marks of frequent scouring. If you must store your multipurpose binoculars in this manner, at least use eyepiece lens caps or a rain guard to keep the dust at bay. Even better, store them somewhere where they are less likely to be dislodged by other household members and are less likely to have to endure the tremendous temperature ranges to which a black object on a sunny windowsill is subjected. Those who store binoculars on windowsills are second only to those who use unmounted binoculars without a neck strap in ensuring the survival of the binocular repair industry!
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